1. Field of the Invention
The invention relates to a true logarithmic amplifier.
2. Description of Related Art
The architecture of true logarithmic amplifiers has for many years consisted of parallel limiting amplifiers 11 and linear amplifiers 12, as shown in FIG. 1. This method provides compression of the input dynamic range and typically the gain of the limiting amplifier is about 10 dB, which provides a reasonable compromise between the conflicting requirements of maximum dynamic range for any given number of stages, and accuracy. The parameters of low and high level gains and limiting level are required to be stable with temperature. Finally, the response time of this is very small, and it is possible to see logarithmic action from cycle to cycle of the input signal providing that the amplifier bandwidth is adequate.
This method has some difficulties in its implementation especially in integrated circuits. Because the linear amplifier must supply the full output swing at the output of a multi-stage amplifier, and it is uneconomic to produce different integrated circuits for each stage, the circuits produced are capable of producing a large output voltage and this requires the power consumption to be increased to cater for this. Further, true logarithmic amplifiers have been used in applications where variations in transmitted signal phase with input signal level must be minimised, and maintenance of this performance requires extremely wide amplifier bandwidth.